If photovoltaic systems are intended to be connected to a power grid, it is necessary to heed a number of requirements that are predefined by a power supply company. With this type of feeding into three-phases, a power difference between the phases must not be greater than 4.6 kVA. A 10-minute average value must have a value of less than 5 kW, for example, in some jurisdictions.
Another requirement imposed by a power supply company is for external voltage monitoring to be carried out for a grid monitoring device (ENS). This makes it possible to comply with a disconnection limit of 110% (for example 253 volts), for example.
It is also important, in the case of an excess supply of power in the power grid, that decentralized feeders can preferably be disconnected by remote disconnection since only as much power as is actually also consumed may be provided in the grid.
Island grid detection is also necessary so that the maintenance personnel are not endangered by voltage that is still present during maintenance work in the grid.
All of these requirements increase the price of a photovoltaic system if the latter is intended to feed power into a public power grid.
DE 10 2006 003 904 A1 presents a photovoltaic system that is used to tackle the load unbalance requirements even with single-phase inverters with more than 4.6 kVA. This solution is also known as a “power balancer”. The system comprises a plurality of single-phase inverters. Each inverter is provided with a failure measuring circuit or an intertripping circuit that is connected to an external communication link. This circuit has the task of detecting failure of an inverter, with the result that the power of the other inverters is reduced in the case of an asymmetrical power feed. Each of the inverters is provided with an automatic isolation point. Current is measured only inside the inverter in order to detect failure. A grid contactor is situated between the inverters and the grid. In this system no measurement is carried out at the grid connection point and an additional central switching element or a grid contactor must be used.
Another design of a photovoltaic system for feeding current into an electricity grid is known from DE 10 2004 025 923 A1. This design comprises a plurality of inverters. A communication bus connects the inverters to one another in terms of data technology, a central control and monitoring device that is likewise connected to the communication bus being provided. A grid monitoring device that at least temporarily disconnects the inverters if predefinable grid monitoring criteria are not complied with is also present. The central control and monitoring device avoids mutual influence and unnecessary disconnection of the inverters. The solution has monitoring carried out only at the central measuring and control point, with the result that a design as a power balancer is not possible. The grid is monitored only with respect to a voltage or a frequency. In addition, only passive island grid monitoring, that is to say three-phase voltage monitoring, is possible. However, this is not permissible in inverters that individually control their phase currents. Therefore, this solution is not suitable for use with some embodiments of three-phase inverters.